Pop the popcorn according to the package’s instructions.
Make the popcorn: Place the popcorn in a gallon-sized resealable bag and add butter, brown sugar, and cinnamon. Seal and shake to coat the popcorn, then pour out on a baking sheet in a single layer.
Make the glaze: Mix powdered sugar and water. Add water by the ½ teaspoon until you reach desired consistency (it should be about as runny as the glaze on a cinnamon roll).
Drizzle glaze on popcorn and refrigerate, 20 to 25 minutes.
